Blood pressure varies with each heart beat, and varies between high and low levels, most commonly known as systolic and diastolic pressures. The monitoring of pressure that blood exerts on the walls of arteries is the most popular prior art method used to determine blood pressure. Pressures are commonly obtained by using a stethoscope and a sphygmomanometer, in the method known as the cuff method. It is not convenient to monitor pressure and/or the change in pressure over a long period of time by use of the sphygmomanometer because it is labor intensive as it entails measuring the blood pressure by comparing the pressure in the main artery of the arm with the pressure in an inflatable cuff wrapped around the arm. The sphygmomanometer restricts muscle movement, is uncomfortable and chafes the skin.
Prior art devices for automatically monitoring blood pressure include U.S. Pat. No. 2,549,049 which describes a diaphragm type blood pressure gage which has a flexible membrane sensor in contact with the skin directly over an artery. The membrane is equipped with a monitor which generates an output related to blood pressure. This method basically requires monitoring a change in the position of the membrane by arterial distension due to variation of blood pressure. A problem arises because it is difficult to accurately position the membrane over the artery, calibrate the device, and account for the effect of tissue elasticity, density and/or skin displacement. In addition, certain drugs reduce blood pressure while at the same time increasing arterial distention.
U.S. Pat. No. 3,704,708, describes a similar device used to measure the displacement of a membrane sensor which is secured in a housing. In use, the membrane is secured to a skeletal portion of the head. The device engages a bony grooved region which encompasses a vascular duct. The housing defines a laterally extending open channel defined by ridged or shoulder portions. The shoulders fit over a complementary vascular duct and engage the bony region about the duct, thereby holding the membrane within a channel and pressing against the duct.
It has been found that it is very difficult to keep a membrane sensor directly over an artery since any body or tissue motion may relatively move the sensor sufficiently to prevent a correct reading from being obtained. Further, there are indications that the artery itself attempts to move when compressed. Once the sensor is shifted from its original position the readings will be affected. In addition, the elasticity of the tissue will vary depending on the proximity of the sensor to the artery and also affect the accuracy of the readings.